This invention relates to an industrial catalyst, its preparation, and its use, especially for the production of EDC by the oxychlorination of ethylene in a fluidizable or fixed bed reactor.
The oxychlorination of ethylene to 1,2-dichloroethane (EDC) is known to be catalysed by catalysts containing, inter alia, copper, suitably in the form of its chloride, often in admixture with alkali metal salts, and carried on an alumina support. Such catalysts have been described, as well as the related preparation methods, in several patents. The ethylene oxychlorination reaction
C2H4+2HCl+xc2xdO2xe2x86x92C2H4Cl2+H2O 
exploits a catalyst whose active phase is copper chloride. The first patents claimed a simple catalyst prepared by impregnating a suitable support, often xcex3-alumina, with a solution containing copper chloride. However, a real development of such catalyst was achieved by adding a further salt, chosen from among the alkali, alkaline-earth or rare-earth metals. Thus, binary, ternary and sometimes quaternary compositions are described in many patents, as well as different preparation methods.
A typical example of a binary composition is described in EP 041330 (PPG) and is a catalyst prepared with copper and potassium chlorides. The claims refer to a real active phase constituted by KCuCl3. The method of preparation is based on a co-precipitation of CuCl2 and KCl on various supports, the preferred one being attapulgite clay. Copper content in the catalyst ranges from 4 to 12% by weight preferably between 7 to 9%. The molar ratio Cu:K is 1:1. Such a catalyst is said to be suitable for fluid bed applications.
European Patent EP 119933 (MONTEPOLIMERI) describes a binary composition based on copper and magnesium, codeposited on the support as chlorides. In this case, the preferred support is xcex3-Al2O3, preferably with a relatively high surface area and proper pore volume. The amount of copper in the catalyst lies in the range 1-10% by weight, while the magnesium content goes from 0 to 1 mole per mole of copper. The preparation method includes the use of HCl during the dry impregnation procedure.
European Patent EP 176432 (AUSIMONT) describes a fluidizable catalyst. Copper and magnesium are the metals used and the crucial item in the preparation method is said to be the radial distribution of the active phase inside the support particles. In fact, a catalyst having less copper on the surface of the particles is claimed, with relevant reduction of sticking phenomena. Copper and magnesium content in the catalyst falls between 1 and 10% by weight, preferably between 2 and 6% for copper.
In EP 0278922 (ENICHEM SYNTHESIS) is described a catalyst for fluid bed applications and the method for its preparation based on xcex3-alumina, Cu and alkali or alkaline earth metals. Copper content ranges from 3 to 7% by weight while from 0.01 to 4% of the aforesaid additives is included. The examples describe a xcex3-Al2O3-supported catalyst containing Cu and Ca; a catalyst containing also Mg; and a catalyst which includes Li instead of Mg. All the catalysts were prepared by one-shot impregnation, with an aqueous solution of the salts. The oxychlorination reactions are carried out using an air-based process, operating with an oxygen excess.
U.S. Pat. No. 4,446,249, (GEON) describes the use of a catalyst containing copper on xcex3-alumina, wherein the support is modified prior to the deposit of copper by incorporating in it from 0.5 to 3.0% by weight, based on the weight of the support, of at least one metal selected from the group consisting of potassium, lithium, rubidium, caesium, alkaline earth metals, rare earth metals and combinations thereof, by admixing a water solution salt of metal(s) with the xcex3-Al2O3, support, drying the mix and calcining it at 350 to 600xc2x0 C. for about 4 to 16 hours. However, in the example in this patent, even though the Cl/C ratio settled is near the stoichiometric value and despite the excess of oxygen (about 60% above the stoichiometric) the HCl conversion to EDC is decidely low with respect to the usual standard conversion required by and achieved in modern industrial plants ( greater than 99%).
U.S. Pat. No. 3,624,170 (TOYO SODA) claims a ternary catalytic composition based on CuCl2, NaCl and MgCl2, the atomic ratio Cu:Na:Mg being 1:0.2-0.7:0.3-1.5. Such catalyst is claimed to avoid the deactivation caused by contamination due to FeCl3 present inside the stainless steel reactors.
EP-A-0255156 (SOLVAY) describes ternary catalytic compositions containing a mixture of copper chloride, magnesium chloride and an alkali metal chloride which is sodium chloride or lithium chloride, used in precise proportions, which enable a good yield to be achieved in a fluidized bed process for the oxychlorination of ethylene to 1,2-dichloroethane, simultaneously reducing the corrosion of stainless steel reactors as a result, in particular, of a reduction in the sticking and clumping of the particles of catalyst. This document teaches that, for ternary compositions containing copper chloride, magnesium chloride and sodium chloride as an alkali metal chloride, a Na/Cu atomic ratio above 0.2:1 leads to problems of corrosion of the reactor. In contrast, if lithium is used as an alkali metal, no corrosion phenomenon is seen over a wide range of Li/Cu atomic ratios. However, the examples show the appearance of problems of sticking and clumping of the catalyst with compositions containing Li in an Li/Cu ratio above 0.6.
U.S. Pat. No. 4,849,393 (GEON) describes catalysts containing, besides copper chloride and an alkali metal salt, a rare earth metal salt. The catalysts contain from about 2% to about 8% by weight of copper, from about 1% to about 10% by weight of a rare earth metal salt and from about 0.25% to about 2.3% by weight of an alkali metal salt. All the salts are co-deposited on a suitable support by means of the dry impregnation procedure, to give a catalyst which allows high ethylene efficiency and low stickiness. In particular, it is stated that, using copper chloride, potassium chloride and one or more rare earth metal chlorides, an excellent catalyst for fluid bed ethylene oxychlorination is obtained.
More specific is the composition of the catalyst claimed in EP A 0375202 (ICI), in which is described a ternary catalytic composition based on copper chloride, magnesium chloride and potassium chloride. Copper content ranges from 3 to 9% by weight, while that of magnesium and potassium is from 0.2 to 3%. The preferred atomic ratios Cu:Mg:K are 1:0.2-0.9:0.2-09.
U.S. Pat. No. 5,260,247 (SOLVAY) describes a quaternary catalytic composition based on CuCl2, MgCl2, LiCl and at least one other alkali metal chloride on an inert support (Al2O3). Also in this patent the support is impregnated with the metal salts in one shot. The examples refer to an air-based oxychlorination process, operating with an oxygen excess of 36% and a Cl/C ratio of 0.95.
The catalytic activity of copper chloride supported on xcex3-alumina towards the oxychlorination of ethylene to EDC is thus well known and it is also known that several alkali metal salts or alkaline-earth metal salts improve the performance of the catalyst in terms of selectivity and productivity in fixed and fluid bed reactors. In the latter case, the fluidization is particularly critical, especially when the Cl/C ratio is close to 1, because in these conditions the HCl excess induces the phenomenon of stickiness. In the recycle process, where the ethylene is in excess with respect to the HCl, the problem of sticking is negligible and the final target is the maximum HCl conversion achievable operating with a low excess of oxygen, thus maximizing the ethylene yield to EDC. This is also true for fixed bed applications, characterized by a reactant stream very rich in ethylene.
As far as industrial fluid bed oxychlorination reactors are concerned, the main problems are related to: fluidization of the catalyst, abrasion of the reactor, ethylene yield to EDC and EDC productivity. In recycle processes, with which the present invention is particularly concerned, fluidisation of the catalyst is not a problem. Even abrasion of the reactor does not represent a critical item, because xcex3-alumina is usually used to prepare catalysts for fluid bed applications. However, any improvement in ethylene yield or in productivity is fundamental for an industrial application. It is, therefore the object of the present invention to provide a catalyst which is suitable for any oxychlorination reaction, air- or oxygen-based, and particularly for the oxygen-based process, which operates with vent gas recycle. Such a catalyst must lead to improved ethylene yield and productivity with respect to the existing industrial catalysts.
According to the present invention there is provided a catalyst which is suitable for catalysing the oxychlorination of ethylene to 1,2-dichloroethane, which comprises a xcex3-alumina support coated with a first layer containing magnesium and, on the first layer, a second layer containing copper and, optionally, lithium.
The invention also provides a process for preparing a catalyst suitable for catalysing the oxychlorination of: ethylene to 1,2-dichloroethane, which comprises impregnating xcex3-alumina with a solution containing a magnesium salt, drying the product, and impregnating the product with a solution containing a copper salt and, optionally, a lithium salt.
The catalyst suitably contains, by weight, from 0.1 to 5%, preferably 0.1 to 2%, magnesium; from 2 to 10%, preferably 2 to 8%, copper; and from 0 to 5%, preferably 0 to 1%, lithium. A particularly preferred catalyst contains, by weight, 0.5 to 1.5% magnesium; 3 to 6% copper; and 0.1 to 0.3% lithium.
The xcex3-alumina used as the catalyst support preferably is one having a surface area of from 50 to 220 m2/g, especially 80 to 180 m2/g, and an average particle size in the range 40 to 60 xcexcm.
In a preferred process for producing the catalyst the xcex3-alumina is dried, in order to remove water adsorbed inside its pores, and is then impregnated with a solution of a magnesium salt, suitably magnesium chloride. The product is dried, suitably overnight, and it is then impregnated with a solution of a copper salt, suitably copper chloride, either alone or, preferably, in combination with a lithium salt, again suitably as lithium chloride. The product of this second impregnation step is then dried.
By pre-impregnating the xcex3-alumina with a magnesium salt the acidic centres on the alumina surface are neutralised, forming magnesium aluminate, and are thus unavailable for reaction with the copper. This means that all or substantially all of the copper is available for catalysing the oxychlorination reaction. This is demonstrated by the following solubility tests the results of which are given in Table 1.
Three catalysts were prepared by impregnating xcex3-alumina with 1) copper chloride, 2) copper chloride and magnesium chloride, and 3) magnesium chloride. The catalysts were treated with acetone, which is able to dissolve CuCl2 and MgCl2, but not copper or magnesium aluminate, nor the copper hydroxo-complexes, such as paratacamite Cu2(OH)3Cl. The catalyst containing only copper had its metal content reduced from 4 to 3.52%, while the presence of magnesium enhanced the amount of free copper chloride, leaving only 2.92% copper on the catalyst. At the same time, due to the competitive reaction between copper and magnesium to form the aluminate, part of the magnesium was present as chloride and dissolved in acetone, leaving 0.58% of that metal on the catalyst. Such competitive reaction is absent when there is no copper on the catalyst. In fact, all the magnesium added becomes insoluble (sample 3). It is to be noted that the amount of magnesium used for preparing the Mg/Al2O3 system was the same as was used for prearing the Cu/Mg/Al2O3 catalyst.
However, to have a catalyst not only active, but also giving a high productivity, a high dispersion of the active phase is required, i.e. very small crystals should be present all over the support surface. Big agglomerates do not help the catalyst action. Co-precipitation of CuCl2 and MgCl2 leads to the formation of macrocrystals, while the presence of lithium chloride produces microcrystals, enhancing the dispersion of the active phase. As a consequence, the preferred preparation method is as follows:
(i) impregnation of xcex3-alumina with a solution containing MgCl2 (dry impregnation procedure);
(ii) drying at 80xc2x0 C. overnight;
(iii) second impregnation with a solution containing CuCl2 and LiCl followed by drying at 80xc2x0 C. overnight;
(iv) activation of the catalyst at 200xc2x0 C.
In experimental trials, the activation may be carried out directly inside the fluid bed pilot reactor.
The synergistic effect due to this preparation procedure provides a very efficient catalyst for ethylene oxychlorination, particularly for the oxygen-based process, which operates by recycling the vent gas and with low oxygen excess and low Cl/C ratios. This synergistic effect represents a substantial advantage over known methods. For example, in U.S. Pat. No. 4,446,249 (GEON), which describes a two-steps preparation procedure where the additive(s) are added prior to copper deposition, the method requires a calcination after the first impregnation, so that the support is modified through a reaction between the support and the alkali, alkaline-earth or rare earth metal salt(s). As a consequence, the support which is impregnated with the copper chloride solution has changed. Moreover, besides copper chloride no other salt is present in the solution used for the second dry impregnation.
On the contrary, the present invention is not based on a bulk modification of the support and a following simple impregnation with copper chloride, but rather modifies chemically only the surface of the support by means of magnesium aluminate formation and at the same time enhances the dispersion of copper, deposited in the second step together with lithium, whose ionic dimension induces the formation of highly dispersed small crystals.